The present invention relates to a photoconductor for electrophotography, hereinafter referred to simply as a "photoconductor", used in electrophotographic apparatuses such as printers, copying machines and facsimiles. More specifically, the present invention relates to a photoconductor that includes a photosensitive film containing a specific bisazo compound as a charge generation agent.
Conventional photoconductors include an electrically conductive substrate and a photosensitive film on the electrically conductive substrate. It is necessary for the photosensitive film to retain surface charges in the dark, to generate charges in response to the received light, and to transport charges in response to the received light. The so-called single-layer-type photoconductor includes a mono-layered photosensitive film that exhibits all the above described functions. The so-called laminate-type photoconductor includes a photosensitive laminate film including a charge generation layer that contributes mainly to charge generation and a charge transport layer that contributes to surface charge retention in the dark and to charge transport under light exposure.
The photoconductive materials for the photoconductor includes inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide, and cadmium sulfide. The selenium film or the selenium alloy film is formed by vacuum deposition. Small grains of zinc oxide or cadmium sulfide are dispersed into an organic solvent, in that a resin binder is dissolved, and the organic solvent is used as a coating liquid. The photoconductive materials for the photoconductor also includes organic photoconductive materials such as poly-N-vinylcarbazole, poly(vinyl anthracene), phthalocyanine compounds and bisazo compounds. The poly-N-vinylcarbazole solution or the poly(vinyl anthracene) solution is used as a coating liquid. A film of a phthalocyanine compound or a film of a bisazo compound is formed by vacuum deposition. Optionally, small grains of a phthalocyanine compound or a bisazo compound are dispersed into an organic solvent, in that a resin binder is dissolved, and the organic solvent is used as a coating liquid.
When a bisazo compound is used as a charge generation agent to form a single-layer-type photoconductor or a laminate-type photoconductor, usually small grains of the bisazo compound are dispersed into an organic solvent, into which an appropriate resin binder is dissolved. Visual defects and image nonuniformity are caused when the bisazo compound grains are not so small enough as to be dispersed uniformly. Various investigations have been conducted on the influences of the kinds and the amounts of the impurities on the grain size and the dispersibility of the bisazo compound.
Among many bisazo compounds, a bisazo compound described by a structural formula (I) (hereinafter referred to as "DCPB") ##STR3##
is used as a charge generation agent that provides the photoconductors with preferable electrical properties such as high sensitivity and a low residual potential (cf Japanese Unexamined Laid Open Patent Application No. S63-305362).
DCPB is synthesized by the method disclosed in Japanese Unexamined Laid Open Patent Application No. H01-282268. In many cases, DCPB is synthesized using a compound described by a structural formula (II) (hereinafter referred to as "PB"). ##STR4##
As described above, it has been known to those skilled in the art that DCPB is a preferable charge generation agent. As a consequence, various investigations have been conducted on synthesis of DCPB and its purification. However, it has not yet been clarified that there exists a certain material that relates closely to the preferable grain diameter of DCPB and its preferable dispersibility which are favorable to obtain a uniform and even coating film of DCPB.